08 14 0 v 1 9 A ug 2 00 0 A Deep ROSAT HRI Observation of NGC 1313

We describe a series of observations of NGC 1313 using the ROSAT HRI with a combined exposure time of 183.5 ksec. The observations span an interval between 1992 and 1998; the purpose of observations since 1994 was to monitor the X-ray flux of SN1978K, one of several luminous sources in the galaxy. No diffuse emission is detected in the galaxy to a level of ∼12×10 ergs s arcmin. A total of eight sources are detected in the summed image within the D25 diameter of the galaxy. The luminosities of five of the eight range from ∼6×10 37 to ∼6×10 erg s; these sources are most likely accreting X-ray binaries, similar to sources obseved in M31 and M33. The remaining three sources all emit above 10 erg s. We present light curves of the five brightest sources. Variability is detected at the 99.9% level from four of these. We identify one of the sources as an NGC 1313 counterpart of a Galactic X-ray source. The light curve, though crudely sampled, most closely resembles that of a Galactic black hole candidate such as GX339-4, but with considerably higher peak X-ray luminosity. An additional seven sources lie outside of the D25 diameter and are either foreground stars or background AGN. Subject headings: Galaxies: spiral; stellar content